1. Field of the Invention
The present invention relates to an image reading apparatus and an image reading method, and a sheet processing apparatus, each configured to read the images of sheets, as the sheets being transported are photographed.
2. Description of the Related Art
Sheet processing apparatuses have been put to practical use, each designed to count and inspect sheets such as securities and bills. The sheet processing apparatus comprises a main unit and a management terminal. The main unit can bundle, seal or shred the sheets according to the results of inspection. The management terminal is connected to the main unit by a cable, such as a LAN cable, and manages the count information the main unit has generated.
Any sheet processing apparatus of this type has an image reading apparatus configured to acquire an image of each sheet in order to inspect the sheet. The image reading apparatus has a sensor, a background plate, and a reading surface. The sensor photographs sheets being transported in a transport path, and read images of the sheets. The background plate is set, facing the reading surface across the transport path. The background plate has a reference-color part (e.g., white part), so that the image reading apparatus may reliably read images from sheets. The sensitivity of the sensor is corrected on the basis of the image of the reference-color part that the sensor has read.
Sheets are not always transported in the same orientation along the transport path. They may skew with respect to the centerline of the transport path. If a sheet skews while being transported in the transport path, its image read from the sheet skewed will not collated with the reference image data stored in the sheet processing apparatus.
Most sheets are made of material having high reflectivity. An image-data reading apparatus that utilizes this fact is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-231311. This image-data reading apparatus has a background plate having a black part (i.e., low-reflectivity part). Having this background plate, the image-data reading apparatus can detect the skew of a sheet, from the image of the sheet it has read.
The sheets any sheet processing apparatus can process are very thin, however. Part of the light applied to each sheet therefore passes through the sheet. In the image-data reading apparatus specified above, the light passing through the sheet is applied to the background plate. The background plate absorbs a part of the light and reflects the remaining part of the light. The light that the background plate has reflected passes through the sheet and illuminates the sensor. That part of the image of the sheet, which represents the black part (low-reflectivity part), is therefore darker than the remaining part of the image, which represents the white part (i.e., high-reflectivity part) of the background plate. Consequently, the image-data reading apparatus cannot acquire correct image data about the sheet even if the sensitivity of the sensor is corrected on the basis of the image of the reference-color part.
The sheet processing apparatus determines whether each sheet is a normal one or a stained one, from the image of the sheet that the image reading apparatus has read. The sheet processing apparatus discriminates any dark part of the image, as stain on the sheet. Inevitably, the apparatus regards the dark part of the image as stain on the sheet, though the dark part, i.e., the low-reflectivity part of the background plate, is not stain.